Difficulties with Environmental Valuation
Non-Market Goods 
Most environmental goods, such as clean air and water, and healthy fish and wildlife populations, are not traded in markets. Their economic value - how much people would be willing to pay for them in dollars is not revealed in market prices. The only option for assigning dollar values to them is to rely on non-market valuation methods.

Non-Rival Goods 
One person's consumption of most goods (apples or housing) reduces the amount available for everyone else. Environmental goods are different. Clean water and air, beautiful views, and to some extent outdoor recreation, can be enjoyed by everyone in the same way as radio and television. The economic value of non-rival or public goods is the sum of all people's willingness to pay.

Non-exclusive Goods 
People cannot be excluded from enjoying most environmental goods and the cost of trying to exclude them is prohibitive. Other than increases in onsite hunting and fishing opportunities, which may be a source of economic benefit to farmers, the environmental benefits of most conservation practices are non-exclusive. The free rider problem makes it impractical for farmers to recoup the cost of on-farm conservation investments from those who benefit from off-farm environmental improvements. 

Inseparable Goods 
Conservation practices at a given site contribute in many roundabout ways to environmental goods and result in environmental and economic benefits that accrue over great distances in time and space. It may be impossible to separate the economic benefits that result from one conservation practice undertaken at one site from another undertaken at another site. Worse, it may be impossible to separate the aggregate benefits of those practices from those of other environmental investments.